The present invention relates in general to distributed power systems and applications thereof and, more specifically, to a synchronous rectifier converter that limits the build up of reverse currents by self-adjustment to a very small percent of the load current so the converter will not have problems in parallel with other modules.
As logic integrated circuits have migrated to lower working voltages in search for higher operating frequencies, and as overall system sizes have continued to decrease, power supply designs with smaller and higher efficiency power modules are in demand. In an effort to improve efficiencies and increase power densities, synchronous rectification has become necessary for these type of applications. Synchronous rectification has gained great popularity in the last ten years as low voltage semiconductor devices have advanced to make this a viable technology.
At the same time, the distributed power system has become popular and critical for both telecommunication and Internet networks. DC-to-DC (DC/DC) converters are a key part of the distributed power systems since they change the high amplitude DC bus voltage to the much lower DC voltage suitable for powering and driving logic level Integrated Circuits (ICs). The supply voltage for a new logic IC changes from 5.0V to lower voltages such as 3.3V, 2.5V, 1.8V, and 1.0V, for example, in order to increase the chip operation speed and density. Furthermore, very high-density board designs only reserve limited space for power supplies.
On the other hand, the IC chips demand much higher supply currents and the power loss of traditional diode rectifiers with high current is so large that thermal management becomes very difficult for power supplies. To meet this great challenge for power supplies, the power loss and heat need to be reduced greatly. The synchronous rectifiers can cut the power loss significantly and increase the power density very effectively because the power loss of MOSFETs is much lower than that of diodes, but the synchronous rectifiers require the proper drive circuit for the MOSFETs, increasing the complexity of synchronous rectifier circuits.
Accordingly, standard modules in parallel are often required to meet the demands of high output current applications. A main advantage is that synchronous rectifiers in parallel can have bi-directional power flow while the diode rectifier can only have power flow in one direction. Synchronous rectifier operation benefits from the two-direction MOSFET because the converter can always work in the continuous mode even at light load but converters may not work properly when modules are in parallel with other modules.
A major limitation of the use of synchronous rectifiers for distributed power systems is that the reverse current to the synchronous rectifiers can render them highly inefficient or damage them entirely. In particular, in reverse power-flow conditions, a large circulating current between modules can develop placing high voltage stresses on the MOSFET, which may damage the converters. Therefore, reverse current must be avoided in order to parallel the synchronous rectifiers.
A prior art method of limiting the effects of reverse currents in a synchronous rectifier arrangement involves the use of a sensing circuit to turn OFF the MOSFET once a reverse current is detected. This approach is illustrated in FIG. 1, which shows the full-bridge DC/DC converter with synchronous rectification. The load current is sensed and used to control the secondary driver. The synchronous MOSFET is turned off when the negative current is detected. This approach, however, requires the use of a load current sensing circuit which adds complexity and cost to the system and requires shutdown of the synchronous rectifier circuit for some time. The operation mode of such a converter is therefore discontinuous at light load levels with the converter oscillating between a continuous mode and a discontinuous mode.
Accordingly, a synchronous rectifier circuit that limits the effects of reverse current with minimal parts and less complexity would be advantageous.
The present invention provides a unique and novel approach to preventing the effects of reverse current in a synchronous rectifier system. With the present invention, the synchronous rectifier circuit has a reduced part count and reduced complexity. Furthermore, the synchronous rectifier of the present invention can apply to both isolated and non-isolated power converters.
Accordingly, disclosed in one embodiment is a synchronous rectifier circuit that is driven in such a way so that the reverse current does not build up and cause damage or limit the efficiency of the circuit. Essentially, the negative current is self-adjusted to a very small percent of load current, so the converter will not have problems in parallel with other modules.
A technical advantage of the invention is that the reverse current is prevented in a natural way so that the negative load current sensing circuit and shutdown circuits are not needed.